1. Field of the Invention
The present invention relates generally to a shadow mask type color cathode ray tube, and more particularly to a shadow mask type color cathode ray tube equipped with a rectangular face panel whose screen has a flattened exterior surface.
2. Description of the Prior Art
When the exterior surface of a screen of a face panel is spherical, the exterior surface gives a more spherical appearance in a large-size color cathode ray tube than in a small-size color cathode ray tube. Thus, a large-size color cathode ray tube has an unnatural appearance in regeneration of pictures. In addition, the reflection of incident light upon the spherical exterior surface is likely to reduce the contrast in brightness in the regeneration picture. In the case of a large-size color cathode ray tube, a wide angle of deflection of at least 110.degree. is required so as to minimize its depth and weight.
In principle, the useful screen of a face panel has an equivalent radius of curvature determined on the basis of the diagonal diameter of the useful screen. Referring to FIG. 5, a rectangular face panel 1 has a screen including a useful screen 2. The center of it is defined as the origin 0, and a horizontal axis passing through the origin 0 and orthogonal to the tube axis z is defined as the X-axis, a vertical axis passing through the origin 0 and orthogonal to the tube axis is defined as the Y-axis. In this way an orthogonal coordinate system is formulated. By using this orthogonal coordinate system, the diagonal diameter of the useful screen 2 is defined as D, and the sagittal height from the origin 0 up to the diagonal radius (D/2) in the z direction is defined as .delta.. The equivalent radius of curvature R.sub.0 of the useful screen 2 of the face panel 1 is expressed as follows: ##EQU1##
Face panels that have an equivalent radius of curvature of about 1.76 times the diagonal diameter D of the useful screen 2, are generally called "1R panels", and face panels having a greater equivalent radius of curvature than those of the 1R panels are called "flat panels".
The shadow mask 3 of a wide angle deflection color cathode ray tube equipped with a flat panel partly domes toward the outside owing to thermal expansion; in FIG. 6, the doming part is indicated by 3a. Such a phenomenon is called "local doming". When it occurs, an aperture 3c of the shadow mask 3 is caused to displace from its proper position 3c to a "false" position 3b as shown in FIG. 6. An electron beam 5a is compelled to reach a phosphor portion 4b, instead of a phosphor portion 4a, through the aperture 3b displaced to the "false" position as a "false" electron beam 5b. If no local doming occurs, the electron beam 5a would reach the phosphor portion 4a through the aperture 3c as designed. The deflection of the electron beam 5a spoils the purity of color.
In order to achieve a flat screen exterior surface in large-size cathode-ray tubes, a thick glass bulb must be used so as to withstand atmospheric pressure after evacuation, thereby increasing its weight.
Even if the face panel has a spherical screen surface and if the peripheral portion is rather flat, the screen surface gives a flat appearance as a whole. The cathode ray tube disclosed in U.S. Pat. No. 4,786,840 takes advantage of this phenomenon. Specifically, this prior art cathode ray tube has a flattened peripheral portion, and the portion extending from the center to the periphery of the useful screen, which is most susceptible to local doming, has especially increased curvature.
Under the last-mentioned prior art cathode ray tube, a sagittal height occurs between the center and the peripheral portion. This causes the inversion of the symbols of quadratic differentials of the spherical surfaces in the diagonal direction, thereby causing a saddle-like bowing, commonly called "inverted bowing". The increase in the curvatures from the center to the periphery along the X axis and Y axis, and the inverted bowing jointly affect the reflection of incident light upon the useful screen of the face panel, thereby producing unnatural reflection. With an image, particularly a moving object, the speed of motion gives an unnatural appearance in an area where the change of curvature is large.
In order to solve this problem, other prior art disclosed in Japanese Laid-Open Patent Publication No. 62-177841 (U.S. Pat. No. 4,777,401) proposes that the peripheral portion is flattened from 1.5R to 1.8R (i.e. an equivalent radius of curvature of 1.5 to 1.8 times the equivalent radius of curvature of 1R), and that the portions of the useful screen of the face panel that extend from the center to the diagonal ends have an equivalent radius of curvature ranging from 1.3R to 1.5R. As a whole, this spherical portion is effectively flattened.
This proposal is advantageous in that the inverted bowing is prevented from occurring in the peripheral portion, thereby ensuring that this portion is non-spherical without having any point of inflection. Another advantage is that the glass bulb can be thin, almost equal to the thickness of a conventional 1R panel, thereby reducing the weight of the face panel. In addition, the reflection of incident light upon the useful screen of the face panel becomes natural, and the movement of electron beams is minimized at the occurrence of local doming. Owing to these merits, the proposed face panel is applied to large-size color cathode ray tubes such as 29-inch, 33-inch, and 43-inch cathode ray tubes.
However, the proposal described above is disadvantageous in that since the curvature is gradually diminished toward the periphery of the screen, no additional flattening is permissible even if that be desirable. In the last-mentioned proposal the peripheral portion can be flattened to the degree of 1.3R to 1.5R but when the size of the face panel is increased, the flattened portion nevertheless has an spherical appearance.